This invention relates to steam turbine technology in general, and to an axial-to-radial flow, low pressure, steam turbine exhaust gas diffuser in particular.
A steam turbine low pressure (LP) section typically includes an inlet domain, multiple turbine stages and an exhaust gas diffuser or hood. The exhaust gas diffuser is often mounted at the last row of rotating blades or buckets, and is formed to include an axial flow inlet and a radial flow outlet. One of the main functions of the exhaust hood is to recover static pressure and to guide the exhaust gas flow from the last stage row of buckets into the condenser. Flow diffusion takes place in the initial section of the low pressure diffuser formed by the steam guide and bearing cone, while the remainder of the low pressure hood features collect the gas flow in a chamber and guide it to the condenser. Diffusers are typically designed with respect to optimized turbine performance which may be measured in terms of maximum possible static pressure recovery.
Static pressure recovery of the low pressure exhaust diffuser depends on the Area Ratio formed by steam guide and bearing cone profiles and on the last stage bucket exit profile. Generally, bearing cone profiles are designed with 10-20 degree negative or positive angles at the diffuser inlet adjacent the last stage row of buckets, relative to horizontal (or to the turbine rotor axis). This traditional bearing cone design, however, forms a sharp corner very close to the last stage buckets, which leads to a larger pressure gradient from the bucket inner hub to the radially outer tip. This larger static pressure gradient has negative impact on last stage bucket efficiency as well as on diffuser static pressure recovery.